Using TortoiseSVN how do I merge changes from the trunk to a branch and vice versa?

I couldn't properly follow the other answers, here's more of a dummies guide...

You can do this either way round to go trunk -> branch or branch -> trunk. I always first do trunk -> branch fix any conflicts there and then merge branch -> trunk.

Merge trunk into a branch / tag

1. Checkout the branch / tag
2. Right-click on the root of the branch | Tortoise SVN | Merge ...
3. Merge Type: Merge a range of revisions | Click 'Next'
4. Merge revision range: Select the URL of the trunk directory that you copied to the branch / tag. Enter the revisions to merge or leave the field empty to merge all revisions | click 'Next'
5. Merge options: I just left these as default | click 'Merge'
6. This will merge the revisions into the checked out branch / tag
7. Then commit the merged changes to the branch / tag

The behavior depends on which version your repository has. Subversion 1.5 allows 4 types of merge:

1. merge sourceURL1[@N] sourceURL2[@M] [WCPATH]
2. merge sourceWCPATH1@N sourceWCPATH2@M [WCPATH]
3. merge [-c M[,N...] | -r N:M ...] SOURCE[@REV] [WCPATH]
4. merge --reintegrate SOURCE[@REV] [WCPATH]

Subversion before 1.5 only allowed the first 2 formats.

Technically you can perform all merges with the first two methods, but the last two enable subversion 1.5's merge tracking.

TortoiseSVN's options merge a range or revisions maps to method 3 when your repository is 1.5+ or to method one when your repository is older.

When merging features over to a release/maintenance branch you should use the 'Merge a range of revisions' command.

Only when you want to merge all features of a branch back to a parent branch (commonly trunk) you should look into using 'Reintegrate a branch'.

And the last command -Merge two different trees- is only usefull when you want to step outside the normal branching behavior. (E.g. Comparing different releases and then merging the differenct to yet another branch)

You should use "merge a range of revision".

To merge changes from the trunk to a branch, inside the branch working copy choose "merge range of revisions" and enter the trunk URL and the start and end revisions to merge.

The same in the opposite way to merge a branch in the trunk.

About the --reintegrate flag, check the manual here: http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-merge.html#tsvn-dug-merge-reintegrate